1. Field of the Invention
The present invention relates to an IFF system including an interrogation device having an antenna operating in the HF or microwave range and having a relatively broad radiation characteristic for radiating coded interrogation pulses and for receiving answer signals, and more particularly to such a system which cooperates with a position finding device whose transmission pulses are radiated by way of a relatively narrow radiation lobe and wherein answer signals may be triggered in an answering device in response to arrival of the transmission pulses of the position finding device.
2. Description of the Prior Art
The German Letters Patent 2,051,916 discloses an IFF system having a primary radar device which cooperates with a secondary radar device. As used herein, the term "primary radar" should be taken to mean position determining radar, while the term "secondary radar" should be taken to mean an identification type radar. In the just-mentioned German Letters Patent, at least one sensor is provided in the range of the answering device to react to the arrival of signals transmitted from the primary radar and which processes this information to the answering device. A coded answer is made available in the answering device which, however, is radiated as an answer signal only after the arrival of a signal transmitted from the primary radar device. The transmission pulses of the primary radar device and the interrogation signals of the secondary radar device are thereby positioned in a fixed given time spacing. It should be pointed out that it has been strictly observed in the case of devices heretofore known that a primary radar transmission pulse does not coincide with a secondary radar interrogation signal.
An identification system for determining if a target is a friend or foe is known from the German published application 2,142,944. First of all, an interrogation of the target is carried out with the help of laser transmitting and receiving systems, in order to determine whether a target is a friend (correct answering) or is a foe. If a target is recognized as a friend, a communication system is switched on which also permits information transmission with the help of the laser transmitting and receiving system. In such a case, a transfer to a certain kind of information transmission is therefore carried out after an identification step has been completed. In both cases, however, the same transmission systems are utilized, namely laser transmitters laser receivers.
In the case of secondary radar systems heretofore known, the radiation lobes for the transmission of interrogation signals and for receiving the answer signals are relatively broad. Therefore, the positioning of the target objects with respect to angles becomes very inaccurate. Accordingly, secondary radar devices usually cooperate with primary radar devices to take advantage of the more favorable angle resolution which can be applied for more accurate locating of targets.
Transmission methods operating with laser pulses provide difficulties, since the transmission rate is too low for many imaginable applications. This is particularly true when high power is required, due to large distances. Furthermore, it is difficult to transmit messages safely with a laser transmitter with little loss of information since the transmission path may be impaired due to rain, fog and water vapor, for example. In addition, shadings may very easily occur and the ranges are relatively small. A transmission system for information using only laser signals is therefore not very reliable in a great many applications. This is primarily true when the environment cannot be clearly identified for the transmission path, which is the case when one is concerned with mobile devices.